Twisted nematic liquid crystal cells have recently found widespread commercial acceptance for applications such as electronic watch faces because of their excellent contrast, low voltage requirements and fast response times. These cells comprise two glass plates having a transparent, conductive, patterned film thereon to act as electrodes, usually rubbed or slant evaporated with a thin film of a dielectric to align the liquid crystal in a direction parallel to the plane of the plates. The plates are assembled so that the liquid crystal molecules at each surface are at right angles to each other. The molecules in the intervening space are aligned intermediate thereto, and thus the layers have a twist throughout the cell. When a voltage is applied to the cell, the layers untwist, affecting the passage of light through the cell which is visible through external polarizers.
The liquid crystal materials useful in twisted nematic liquid crystal cells must be stable, have low voltage requirements, fast response times, broad useful temperature ranges and good contrast. One long standing problem in this art is that the range of angles over which good contrast is visible to the viewer is somewhat limited, generally on the order of about 50.degree.-60.degree.. It would be desirable, particularly for larger displays, to maximize the range of angles over which the liquid crystal display is visible.
One known method of increasing the viewing angle is by increasing the ratio of the voltage applied to the cell to the threshold voltage, that is, the minimum voltage needed to realign the liquid crystal molecules in the cell. However, generally this is impractical, particularly in watches, where only small batteries with a fixed voltage output are employed at the source of current. Thus it would be desirable to increase the viewing angle of the liquid crystal cells without increasing the voltage requirements.
It is also known that larger angles of viewability in liquid crystals are found in compositions that have a small index of refraction anisotropy, see Gharadjedaghi and Robert, Revue de Physique Applique, Vol. 11, July 1976. Index of refraction anisotropy is the difference between the index of refraction measured along the principal axis and the index of refraction measured perpendicular to the principal axis. The viewing angle is inversely proportional to the square root of the index of refraction anisotropy.
However, most liquid crystal mixtures in general use either have high index of refraction anisotropies, or are otherwise inadequate because their useful temperature ranges are too high or too small, or their response times are too long.